Rotary cutting tools, such as drills or milling cutters, have a generally cylindrical body and a plurality of sharp cutting edges. The cutting edges are not surfaces as such, but sharp lines formed by the intersection of two other surfaces, specifically an axial flute face ground along the length of the tool and a generally planar relief surface, also known as a flank face. There are as many cutting edges as there are pairs of these intersecting surfaces. The axial flute faces, also known as rake faces, lead the sharp cutting edges, while the flank faces trail the cutting edge. As the cutting edges cut into a metal workpiece surface, chips form and curl continually along the rake faces, moving axially along the flutes and clear of the cutting interface. The rake face especially is subject to wear by the chips, and is also subject to buildup from workpiece material that can literally weld itself to the surface. Consequently, it is desirable to coat the cutting tool surfaces with a protective, wear reducing layer.
One of the most common cutting tool base materials is tungsten carbide, often referred to simply as carbide. Carbide is not a practical material for the entire cutting tool body, and will usually constitute just an insert or cutting tip that is brazed or otherwise secured to a less brittle steel shank. Coatings typically used to protect the carbide fall into two broad categories in terms of how they are applied, physical vapor deposition, abbreviated as PVD, and chemical vapor deposition, or CVD. Physical vapor deposition, a term broad enough to encompass evaporation, ion plating and sputtering, is the lower temperature process of the two, although by no means is it a low temperature process, occurring at around 800 degrees F. A typical material for a PVD process would be titanium nitride. In CVD, temperatures may be more than twice as high. The material does vaporize to a molecular state at the higher temperature, so a chemical reaction does occur on the base material surface to be coated, giving a much tougher and thicker coating. Coating materials may include titanium carbide, aluminum oxide, and others.
A real drawback of the CVD process is that the higher temperatures may have a deleterious effect on the base material. The surface of the carbide, for example, may be decarburized and embrittled, especially where there is a sharp, exposed cutting edge. Another problem is that the CVD coating may build up excessively at the cutting edge, effectively rounding and dulling it. Furthermore, where a carbide insert is brazed to a steel shank or cutter body, the braze seam can melt. Therefore, the conventional wisdom is that a CVD process cannot be used in any cutting application where very sharp cutting edges are needed.